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Exploration of Near Infrared-Emissive Colloidal Multinary Lead Halide Perovskite Nanocrystals using an Automated Microfluidic Platform Ioannis Lignos, Viktoriia Morad, Yevhen Shynkarenko, Caterina Bernasconi, Richard M. Maceiczyk, Loredana Protesescu, Federica Bertolotti, SUDHIR KUMAR, Stefan T. Ochsenbein, Norberto Masciocchi, Antonietta Guagliardi, ChihJen Shih, Maryna I. Bodnarchuk, Andrew J. deMello, and Maksym V. Kovalenko ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b01122 • Publication Date (Web): 12 May 2018 Downloaded from http://pubs.acs.org on May 13, 2018
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Exploration of Near Infrared-Emissive Colloidal Multinary Lead Halide Perovskite Nanocrystals using an Automated Microfluidic Platform Ioannis Lignos,1† Viktoriia Morad,1,2,3 Yevhen Shynkarenko,2,3 Caterina Bernasconi,2,3 Richard M. Maceiczyk,1 Loredana Protesescu,2,3 Federica Bertolotti,4,5 Sudhir Kumar,1 Stefan T. Ochsenbein,2,3 Norberto Masciocchi,4 Antonietta Guagliardi,6 Chih‐Jen Shih,1 Maryna I. Bodnarchuk,3* Andrew J. deMello,1* and Maksym V. Kovalenko2,3* 1
Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir‐Prelog‐Weg 1, 8093, Switzerland
2
Institute of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, Vladimir‐Prelog‐Weg 1, 8093, Switzerland
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Empa‐Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, 8600, Switzerland
4 Dipartimento di Scienza e Alta Tecnologia and To.Sca.Lab, Università dell’Insubria, via Valleggio 11, I‐22100 Como, Italy 5
Aarhus Institute of Advanced Studies (AIAS), Aarhus University, Høegh‐Guldbergs Gade 6B, 8000 Aarhus C, Denmark. 6
Istituto di Crystallografia and To.Sca.Lab, Consiglio Nazionale delle Ricerche, via Valleggio 11, I‐22100 Como, Italy
KEYWORDS: perovskites, microfluidics, nanocrystals, formamidinium, quantum dots, halides ABSTRACT: Hybrid organic‐inorganic and fully inorganic lead halide perovskite nanocrystals (NCs) have recently emerged as versatile solution‐processable light‐emitting and light‐harvesting optoelectronic materials. A particularly difficult challenge lies in warranting the practical utility of such semiconductor NCs in the red and infrared spectral regions. In this context, all three archetypal A‐site monocationic perovskites—CH3NH3PbI3, CH(NH2)2PbI3, and CsPbI3—suffer from either chemical or thermodynamic instabilities in their bulk form. A promising approach towards the mitigation of these challenges lies in the formation of multinary compositions (mixed cation and mixed anion). In the case of multinary colloidal NCs, such as quinary CsxFA1–xPb(Br1–yIy)3 NCs, the outcome of the synthesis is defined by a complex interplay between the bulk thermodynamics of the solid solutions, crystal surface energies, energetics, dynamics of capping ligands and the multiple effects of the reagents in solution. Accordingly, the rational synthesis of such NCs is a formidable challenge. Herein, we show that droplet‐based microfluidics can successfully tackle this problem and synthesize CsxFA1–xPbI3 and CsxFA1–xPb(Br1–yIy)3 NCs in both a time‐ and cost‐efficient manner. Rapid in‐ situ photoluminescence and absorption measurements allow for thorough parametric screening, thereby permitting precise optical engineering of these NCs. In this showcase study, we fine‐tune the photoluminescence maxima of such multinary NCs between 700 and 800 nm, minimize their emission linewidths (to below 40nm, and maximize their photoluminescence quantum efficiencies (up to 89%) and phase/chemical stabilities. Detailed structural analysis revealed that the CsxFA1– xPb(Br1–yIy)3 NCs adopt a cubic perovskite structure of FAPbI3, with iodide anions partially substituted by bromide ions. Most importantly, we demonstrate the excellent transference of reaction parameters from microfluidics to a conventional flask‐based environment, thereby enabling up‐scaling and further implementation in optoelectronic devices. As an example, CsxFA1–xPb(Br1–yIy)3 NCs with an emission maximum at 735 nm were integrated into light‐emitting diodes, exhibiting high external quantum efficiency of 5.9% and very narrow electroluminescence spectral bandwidth of 27 nm.
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Lead halide perovskites of the APbX3 type, where A can be methylammonium (MA, CH3NH3+), formamidinium (FA, CH3(NH2)2+), inorganic cations (Cs+, Rb+), or a mixture thereof and X is a halide (Br, I, and their mixtures) have attracted enormous attention after they were recognized as efficient thin‐film absorber materials for photovoltaics, with power conversion efficiencies of up to 22.7%.1‐4 Recently, there has been a surge of studies on the nanoscale counterparts of these perovskites, and in particular, on colloidal nanocrystals (NCs),5‐12 which hold great promise as versatile photonic sources for displays, lighting, light‐emitting diodes (LEDs), lasers13‐16 and as light harvesters for solar cells and photodetectors.1, 17 Unlike other forms of APbX3 perovskites ‐ bulk single and polycrystals, thin films and other substrate‐grown structures – colloidal NCs have a particular set of advantages, foremost of which is their versatile solution processability and miscibility with other materials, as well as access to quantum‐size effects and their facile surface‐ and shape‐engineering.18 To date, essentially all work on colloidal APbX3 NCs has concentrated on those compositions, which emit in the visible region of the electromagnetic spectrum (between 400 and 700 nm), with CsPbBr3 and FAPbBr3 NCs, exhibiting green photoluminescence (PL, between 500 and 550 nm), being by far the most popular targets. This can be attributed to the high durability of these bromides, as compared to other compositions in the APbX3 family. In the current study, we focus our attention on multinary perovskite NCs of the quinary composition CsxFA1–xPb(Br1–yIy)3, with a view to controlling their optical properties in the near‐infrared (700–800 nm). It should be noted that there is extensive literature regarding red and near‐infrared emissive colloidal MAPbI3 NCs; References 7, 19‐23 being representative examples. However, MAPbI NCs 3 suffer from severe instabilities caused by humidity, light and heat, and eventually decompose into CH3NH2, PbI2, HI, and I2.22, 24‐25 Fully inorganic CsPbX3 NCs (X = Cl, Br, and I), synthesized by a hot injection method, exhibit tunable emission between 410 and 700 nm and high PL quantum efficiencies (50%–90%).6 Presently, CsPbX3 NCs are the focus of attention with respect to their chemical engineering (i.e. identification of precursors, growth kinetics, shape‐control, post‐synthetic reactivity, and up‐ scaling),20, 26‐45 surface chemistry,8, 46‐50 photophysics (single‐dot spectroscopy, lasing etc.),51‐59 and applications in television displays,6, 60‐62 light‐emitting devices,50, 63‐68 and solar cells.17 The usability of red‐emissive CsPbX3 NCs is, however, strongly limited by the phase instability of the 3D polymorphs of CsPbI3. For similar reasons, iodide‐rich CsPb(Br/I)3 compositions are also unstable. As for FA‐ based analogs, which benefit from the higher chemical stability of the FA ion (compared to that of MA+), one‐pot colloidal syntheses have led to the development of stable and highly emissive FA‐doped CsPbI3 NCs (PL peak at ca. 690 nm; ca. 10% FA) and FAPbI3 NCs (PL peak at ca. 780 nm),69 with cubic or nearly cubic NC shapes and mean particle sizes between 10 and 20 nm. Interestingly, unlike colloidal FAPbI3 NCs, bulk FAPbI3 is completely unstable
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due to a phase transition from semi‐conductive perovskite into a yellow, non‐perovskite phase (an observation that will be further discussed in the Results and Discussion section of this study). A formidable challenge with iodide‐ based NCs lies in discovering the compositional space and related synthesis parameters that would provide for continuous coverage of the 700–800 nm spectral range with narrow PL linewidths and without compromising the chemical durability of the NCs. Our choice of the quinary composition, CsxFA1–xPb(Br1‐yIy)3, was motivated by previous thin‐film and bulk single crystal studies,70‐74 which indicate that the concomitant incorporation of Cs and Br into the FAPbI3 lattice stabilizes the 3D perovskite phase and allows for the compositional tuning of bandgap energy through the formation of mixed‐ halide solid solutions.75 With monohalide FAPbI3 and FA0.1Cs0.9PbI3 NCs, the only way to access the entire 700– 800 nm range is to exploit quantum‐size effects and reduce NC size to the 3–8 nm range. Such small NCs are highly labile morphologically and their PL characteristics are consequently broad and unstable.76 In the case of NCs, accessing suitable reaction parameters for the formation of stable CsxFA1–xPbX3 NCs with tunable emission maxima in the range of 700–780 nm is simply not possible with flask reactions.69 Specifically, in a recent study on the formation of mixed‐anion FAPbX3 NCs, it was reported that variation in the Br‐I content led solely to two stable compositions, one emitting at 680 nm and the other at 760 nm.69 This is due to the fact that in NCs, the ability to form certain compositions is governed not only by the thermodynamics of the mixed‐ion phases, but also by the NC surfaces (i.e. surface energy and ligand binding) and chemical equilibria with the precursors in the solution. These factors greatly expand relevant parametric space beyond the mixing ratios of the Cs, FA, Pb, and halide precursors, thus making thorough exploratory synthesis and optimization virtually impossible with conventional flask‐based techniques. Recently, we showcased the potential of droplet‐based microfluidics combined with on‐line absorption and PL spectroscopy in discovering the optimal synthesis parameters for ternary and quaternary (mixed‐anion) CsPbX3 NCs77 and FAPbX378‐79 NCs. The next logical step, which is pursued in this work, is the exploration of NCs of higher compositional complexity, i.e., CsxFA1‐xPb(Br1‐yIy)3 NCs. In such cases, the use of high‐throughput automated microfluidic reactors becomes an absolute necessity for rapid and detailed experimentation. Using microfluidics, we herein demonstrate the formation of CsxFA1–xPb(Br1–yIy)3 NCs with PL peak tuning between 690 and 780 nm and narrow PL linewidths (expressed as full width at half maximum, FWHM). Characterization by X‐ray diffraction (XRD) and elemental analysis pointed to the the incorporated quantities of Cs (0.1%–1.2% with respect to FA) and Br (10%–18% with respect to I). The obtained CsxFA1–xPb(Br1–yIy)3 NCs were approximately cubic in shape, with a mean size of 8–13 nm. The reaction parameters were fully transferrable from our microfluidic platform (pL‐nL scale) to conventional reaction flasks (mL scale). This
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allows us to demonstrate the optoelectronic utility of these NCs by fabricating near‐infrared LEDs with an external quantum efficiency (EQE) up to 5.9% at 735 nm.
RESULTS & DISCUSSION Formability of APbI3. To obtain red to near‐infrared emissive CsxFA1–xPb(Br1–yIy)3 compositions, the structural chemistry of these perovskites must be considered ‐ only those polymorphs of APbI3 compounds that retain 3‐ dimensional (3D) corner‐shared interconnections of lead halide octahedra are potent semiconductors. The extended electronic structure in lead halide perovskites arises from the periodic 3D network of PbX6 octahedra. Low‐ dimensionality polymorphs of the same composition exhibit higher (by at least 1 eV) and often indirect bandgaps, typically on the order of 300–500 nm. 3D polymorphs of all archetypal ternary APbI3 compounds have stability issues. MAPbI3 is chemically unstable, whereas the 3D polymorphs of CsPbI3 and FAPbI3 are thermodynamically metastable and undergo transitions into 1D polymorphs (Figure 1).76, 80‐85 Interestingly, the thin film and nanoscale forms of CsPbI3 and FAPbI3 exhibit extended, but still finite stability in their 3D polymorph forms (from days to several months), primarily due to surface effects.6, 32, 69, 86‐88 Thermodynamic instability is caused by the Cs and FA ions being, respectively, slightly too small and too large for the voids in between PbI6 octahedra. This has been broadly discussed in the literature in terms of the Goldsmith tolerance factor (t) and octahedral factor (µ), which describe the optimal dense packing of charged ions in an ideal cubic 3D perovskite.72, 89‐93 Mixing larger and smaller ions at the A‐site is a powerful strategy for adjusting the geometric fitness of the A‐cation in the void space of the 3D lead halide framework, thereby improving its phase stability. A high entropy of mixing is also considered to be a stabilizing factor.71 In fact, the best phase stabilities and optoelectronic performances have been reported for thin films with such mixed A‐site occupations: FA/MA,94‐97 Cs/MA,98 Cs/FA,70‐73 Cs/MA/FA,99 or even Rb/Cs/MA/FA.4 Towards multinary APbX3 perovskites: the goal of this investigation. Covering the desired spectral range of 700– 800 nm requires broad mapping in Cs‐FA perovskites, because 3D CsPbI3 and FAPbI3 exhibit bandgaps of 1.75 eV (710 nm) and 1.48 eV (840 nm), respectively. From the viewpoint of structural chemistry, it remains unclear which crystal structures will be adopted or are stable when Cs:FA and Br:I ratios are adjusted simultaneously, as the parent Cs and FA compounds have different 3D polymorph structures (Figure 1). We recently found that for bulk single crystals, in accordance with previous studies on thin films starting with ‐FAPbI3, one can concomitantly introduce
up to 15% Cs and 30% Br, whilst maintaining the same cubic crystal structure as ‐FAPbI3.74 As for colloidal NCs, we recently reported the synthesis of a mixed‐cationic composition, Cs0.9FA0.1PbI3, with the same crystal structure as orthorhombic 3D γ‐CsPbI3, via a flask‐based process.69 Multiple variations of the Cs:FA reagent ratio and synthesis temperatures led to the same composition (with ca. 10% FA). From these studies, it can be surmised that the exploration of complex compositions is prohibitively difficult via flask‐based syntheses. This is because with one iteration of only one parameter per synthesis (each lasting several hours when conducted manually), several years will be required to properly map compositional space and other factors, such as the effects of ligands, solvents, and solvation equilibria.
Figure 1. Formabilities of the 3D and 1D polymorphs of CsPbI3 and FAPbI3 compounds and the goal of this study – near‐ infrared emissive LHP NCs. The PbI6 octahedra of ‐FAPbI3 NCs are assembled in a 3D cubic metastable lattice, which spontaneously converts into a 1D hexagonal version (non‐ luminescent) at room temperature. In the case of CsPbI3, the PbI6 octahedra of FAPbX3 NCs are assembled in a 3D orthorhombic metastable lattice (γ‐phase), which eventually converts at room temperature into a 1D orthorhombic δ‐phase (non‐luminescent). The goal of this study is highlighted with a question – can high‐throughput microfluidic screening identify the existence of stable multinary CsxFA1–xPb(Br1–yIy)3 phases in the form of colloidal NCs, which cover the PL region of 700–800 nm, i.e., in‐between ternary 3D‐phases (CsPbI3 and FAPbI3)? We note that bulk ‐FAPbI3 emits at 840 nm and γ‐CsPbI3 emits at 710 nm, whereas their NC counterparts are commonly reported to emit at ≤700 nm and ≤780 nm, respectively.6, 8‐12, 69 It is also noted that the space groups reported for the γ‐ and δ‐phases of CsPbI3 do not differ (while their structures manifestly do) as they can easily be interconverted by simple axis permutations. We used the original Pbnm and Pnma for the γ‐ and δ‐forms, respectively, to maintain consistency with past literature.
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Figure 2. (Left) Illustration of the segmented‐flow reaction platform equipped with on‐line PL and absorbance modules for the synthesis and real‐time monitoring of CsxFA1–xPbX3 perovskite NCs. The microfluidic platform allows for a systematic and independent variation of precursor molar ratios, such as Cs/Pb, FA/Pb, Cs/FA and Br/I, growth times (determined by the flow rate and tube lengths) and temperature. Droplets are generated by adjusting the flow rates of the carrier phase (50−200 µL/min) and that of the dispersed phase (1.2−50 µL/min). (Right) Illustration of a typical flask‐based hot‐injection synthesis of CsxFA1–xPbX3 NCs. Overall, synthesis optimization was performed by mutual information exchange between flask‐based experimentation (identification of suitable precursors, solvents and capping ligands) and microfluidics (optimization of the reaction parameters). The optimized reaction parameters were successfully transferred from microfluidics back into flask reactors, followed by up‐ scaling and additional post‐synthetic characterization (XRD, electron microscopy and stability tests).
Figure 3. Microfluidic synthesis of CsxFA1–xPbI3 NCs. Variation in the (a) PL spectra, (b) FWHM and (c) PL maximum as a function of temperature for Cs0.03FA0.97PbI3 NCs (with the variation in the Cs/FA molar ratio is indicated). Other parameters were as follows: FA/Pb = 9.3, Cs/Pb = 0.3 and reaction time = 10 s. (d–f) Temporal evolution of the normalized on‐line PL spectra, PL maxima and FWHM of Cs0.02FA0.98PbI3 NCs at 80 °C.
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Experimental design and combinatorial strategy. Microfluidic reactors allow for the addition of multiple reagents in a user‐defined manner, rapid thermal and mass transfer and quantitative kinetic investigation of reactions, thus defining an ideal medium for preparing semiconductor NCs with well‐defined morphologies and physicochemical properties.100‐103 In addition, the advances in robust microfluidic configurations,100, 103 real‐time detection methods,104‐111 continuous purification112‐113 and ligand‐exchange114 systems and optimization algorithms115‐ 117 make microfluidic reactors ideal for the detailed investigation of rapid and complex reaction kinetics77‐78, 106, 118 and for the discovery of multicomponent semiconductor NCs.77‐78, 119 Herein, we modified and applied a previously developed microfluidic platform which had been used for the synthesis and real‐time characterization of binary chalcogenide NCs106 and CsPbX3 NCs.77 This platform (see Figure 2 and associated description in the Methods section) incorporates a multiphase microfluidic reactor with integrated PL and absorption detection to rapidly screen reaction conditions. The controlled injection of precursor solutions (Cs‐oleate, FA‐oleate, PbX2 and PbY2) and carrier fluid is performed in an automated manner (using syringe pumps) allowing for the formation of nanoliter droplets using a 7‐port manifold and efficient mixing of precursors (in 300 ms).77 In the case of CsxFA1– xPbX3 (X = Br and I) NCs, we defined four interdependent molar ratios, which were adjusted during synthesis: FA/Pb, Cs/Pb, Br/I and Cs/FA. In this report, the latter is presented as the Cs percentage relative to the FA content. A tube‐ based microfluidic reactor allows for rapid heating of the droplets (within a few hundred ms), along with the real‐ time extraction of PL and absorption characteristics at various reaction times (0.1–20 s) and temperatures (25–130 C). Additional characterization of the synthesized NCs by transmission electron microscopy (TEM) and XRD was conducted in an off‐line manner, by collecting a sufficient quantity of the sample during synthesis at a fixed set of reaction conditions (see the Methods section for details). The optimized parameters were then transferred to conventional flask‐based reactions. Synthesis of CsxFA1–xPbI3 NCs. In our previous study, we showed that FAPbI3 NCs can grow even at room temperature,78 while CsPbI3 NCs with a 3D phase are formed at temperatures above 100 °C.77 Accordingly, we decided that analyzing the temperature range suitable for the formation of CsxFA1–xPbI3 NCs was an important initial task. Figures 3a–c report the variation in emission linewidth and PL peak between 25–130 C (FA/Pb = 9.3, Cs/Pb = 0.3, and %Cs = 3.0). The formation of FAPbI3 NCs takes place at room temperature, which can be inferred by the emergence of the PL peak at 792 nm, consistent with a previous flask‐ based synthetic study, in which 15 nm FAPbI3 NCs exhibited a PL peak at 780 nm.69 Higher temperatures led to a rapid increase in the bandgap energy, which we ascribe to the incorporation of Cs. The possibility of smaller NC sizes (quantum dots 3–10 nm in diameter) causing larger bandgaps can be discounted using post‐synthesis TEM
images; with all NCs obtained in this study for growth times greater than 7 s being 15–20 nm in size. Size evolution occurs very quickly and over the course of several seconds. From 50 to 90 °C, the PL peak remained stable at 740–745 nm (Figure 3b), but with a gradually decreasing FWHM. Higher synthesis temperatures (>110 °C) resulted in PL peaks closer to 700 nm, most likely due to the formation of ternary CsPbI3 NCs with or without minimal incorporation of FA ions. Accordingly, we concluded that the 50–90 °C range is ideal for compositional engineering purposes. The high speed of formation of Cs0.02FA0.98PbI3 NCs is on par with FAPbX3 (see Reference 78 and Figure S1) and CsPbI3 (reported previously);77 taking only a few seconds to stabilize the PL maximum at 740 nm and the FWHM at 52 nm (Figures 3d–f ). Off‐line optical characterization after synthesis indicates that there is no subsequent growth or other form of evolution in the CsxFA1–xPbI3 NCs (see Figure S2). Based on these observations, further rapid automated screening, at a rate of 100 adjustments per synthesis parameter per hour, was carried out with reaction times of at least (and typically) 7 s. The efficiency of Cs incorporation is expected to depend not only on temperature, but also on the solvation conditions and Cs/FA ratio, and to some extent on the Cs/Pb and FA/Pb ratios. Because these relationships are not fully and rationally predictable when the equilibrium constants and involved energies (lattice energies for all compositions, solvation energies, surface energies and ligand binding energies) are not completely known, they were tested in this study in a combinatorial fashion (Figures S3–S6). In brief, for excess FA (by factor of 4 with respect to Pb), a narrow FWHM can be obtained (Figure S3). At much higher FA/Pb ratios (>13), the crystal phase of the NCs tends to change from black to yellow within hours of synthesis. The TEM images revealed severe morphological irregularities in such NCs, which were in the form of large populations of micron‐sized needles and rods (Figure S4). Furthermore, the PL tunability of CsxFA1–xPbI3 NCs was limited to Cs/Pb ratios lower than 2 and Cs loadings 10% (Figure S3). Outside this window, the PL maxima were always in the range of 680–700 nm, suggesting the formation of pure CsPbI3 NCs or their mixtures with other compositions (seen as multi‐Gaussian PL lines; see Figure S3e, for Cs content equal to 23.3%) or perhaps even mixtures of various shapes. The TEM images (Figure S5) illustrate how the three interlinked ratios affect the morphology of the synthesized CsxFA1–xPbI3 NCs. Progressive addition of 0.3%–5.2% Cs+ (with respect to the FA content) continuously tunes the PL maximum from 758 to 710 nm (Figure S6), whilst maintaining a narrow FWHM in the range of 48–55 nm. Although the XRD results suggest that mixed perovskites adopt a structure similar to that of pure FAPbI3 (Figure S7), the majority of CsxFA1–xPbI3 NC samples exhibited low colloidal and chemical stabilities, except those samples with both low FA/Pb ratios (≤ 7) and low Cs loadings (up to 2%). Synthesis of CsxFA1–xPb(Br1–yIy)3 NCs. The addition of Br into the system was then explored as a way of stabilizing crystal structure and tuning the PL maximum in the desired
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range (700–800 nm). The operating temperature and reaction times were similar to those used for the synthesis of CsxFA1–xPbI3 NCs. As the addition of a second halide further increases the complexity of the synthetic system and can shift the product equilibrium towards other perovskite compositions, parametric screening must be performed with caution. We therefore broadly explored the influence of the interdependent molar ratios of Cs/Pb, FA/Pb and Br– content on the optical properties and stability of the Br/I mixtures (as illustrated in Figures S8– S10). In brief, such a combinatorial study revealed that the parametric zones of the FA/Pb and Cs/Pb molar ratios, able to tune the PL peak between 690–780 nm (whilst maintaining a satisfactory FWHM), were 2.5–6.0 and 0.01– 0.04, respectively (Figure S9). In addition, Br loading of up to 15% leads to a linear blue shift of the emission band at all FA/Pb molar ratios (Figure S10). A key message here is that in nearly all optimized compositions, Br addition does not alter the emission linewidth or emission intensity, suggesting that the synthesized NCs have stable optical characteristics. Furthermore, an increase in Br– loading over 25% will deliver perovskite NCs with emission energies in the range of 650–720 nm. However, such an increase in Br– loading can trigger the formation of other perovskite structures, such as FAPb(Br1–yIy)3 NCs, due to excess FA‐ oleate in the reaction system (Figure S11).
Figure 4. (a) PL spectra of colloidal CsxFA1–xPb(Br1–yIy)3 NCs synthesized using the microfluidic platform and exhibiting composition‐tunable bandgap energies between 690 and 780 nm with FWHM values of 40–65 nm and (b) representative on‐line PL and in‐line absorption spectra at different quantities of Cs+ and Br– in the reaction mixture.
Figure 4a presents selected PL spectra in the range of 690 to 775 nm, with FWHM in the range of 45–65 nm. Such precise PL tuning is achieved through a systematic variation of all three interlinked molar ratios within their refined parametric zones. In particular, variation in Cs (in the range of 0.2%–5.2%) and Br (between 0%–15% of the total halide concentration) content leads to a blue shift in the in‐line (i.e. post heating and when the reaction was quenched) absorption and PL spectra (Figure 4b). Most importantly, the incorporation of up to 15% Br– into the structure of CsxFA1–xPbI3 NCs increased the period of stability of the CsxFA1–xPbI3 NCs from several hours to several weeks (Figure S12). Transfer to flask‐based synthesis. To assess whether the optimal parameters can be transferred to conventional
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flask reactors, we carried out hot‐injection synthesis of both CsxFA1–xPbI3 and CsxFA1–xPb(Br1‐yIy)3 NCs (further details are provided in the Methods section). Briefly, to synthesize NCs with the nominal composition of Cs0.01FA0.99Pb(Br0.11I0.89)3 (Figure 5), PbI2 (55 mg, 0.12 mmol) and PbBr2 (5 mg, 0.014 mmol) were suspended in 1‐ octadecene (ODE, 4.6 mL), heated to 60 °C, and then dried in vacuum for 30 minutes. Subsequently, the reaction mixture was heated to 110 °C in a nitrogen environment, followed by the addition of dried solvents – oleylamine (OLA, 0.5 mL) and oleic acid (OA, 1.0 mL). Once the PbI2 dissolved, the reaction mixture was cooled to 80 °C. At this point, a mixture of FA oleate (4.8 mL) and Cs oleate (1.2 mL) stock solutions was injected into the reaction flask. After 5 seconds, the reaction was quenched. The crude solution was then centrifuged and the supernatant was discarded. The precipitate was dissolved in hexane and the resulting solution was centrifuged once again, after which the supernatant and precipitate were separated. The particles from both fractions, supernatant and precipitate, werefurther washed to remove excess organic ligands (see further details in the Methods section). This synthesis procedure yielded nearly cubic NCs with a PL peak at 735 nm (Figure 5a–c), a PL FWHM of 40 nm (after isolation and purification), and a PL QY of 80‐89%. QY drops to ca. 50% in the solid‐state form (NC film). We note that the larger FWHM values detected in‐line in microfluidics can be attributed to size‐fractioning that occurs during isolation and purification. Crystal structure. To uncover the structural details of CsxFA1–xPb(Br1–yIy)3 NCs, synchrotron X‐ray total scattering measurements were performed (using an octane solution of NCs in a quartz capillary, Figure 5c) at the X04SA‐MS4 Powder Diffraction Beamline of the Swiss Light Source (Paul Scherrer Institute, Villigen, CH).120 A combined Rietveld and total scattering approach based on the Debye Scattering Equation (DSE, accounting for structure, size, and anisotropic morphology)121 was used for structural and microstructural characterization of CsxFA1–xPb(Br1–yIy)3 NCs (details are provided in the Supplementary Information). The analysis results suggest a cubic structure in which X– anions are disordered in four equivalent sites around the equilibrium position (inset of Figure 5c), similar to the disorder seen in FAPbI3 and FAPbBr3 NCs.69, 122 The atomic displacement parameters (in the form of the Debye‐Waller factor) were refined for all atoms; the anomalously high values for halides in the un‐split cubic arrangement suggest local structural disorder. The graphical outcomes of the DSE‐based analysis of CsxFA1– xPb(Br1–yIy)3 NCs are summarized in Figures 5c, S13 and S14. In a similar manner to CsPbX3 NCs,12 the peak positions slightly deviate from the cubic metric. Nevertheless, the hybrid NCs investigated here exhibited a different kind of structural defectiveness, which needs further investigation. In addition, to further validate our findings, we compared the experimental data with a Pnma orthorhombic structure model (Figure S15).123 The Pnma structure does not reproduce the experimental peak intensities; at the same time, new
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Figure 5. Optical absorption and PL spectra of CsxFA1–xPb(Br1‐yIy)3 NCs synthesized in conventional flask reactors, exhibiting a FWHM of 40 nm. (b) Bright‐field scanning TEM (STEM) image of CsxFA1–xPb(Br1‐yIy)3 NCs. (c) Synchrotron XRD pattern (black) and best fit (red, 2θ range of 0.5°–130°; λ = 0.563729 Å) for CsxFA1–xPb(Br1–yIy)3 NCs, yielding a refined lattice parameter (a = 6.3296 Å) and the anionic composition. The inset illustrates the cubic perovskite structure of CsxFA1–xPb(Br1‐yIy)3 NCs (space group Pm‐ 3m, with y = 0.87 and x = 0), in which the perovskite framework consists of PbX6 units sharing the octahedral corners; the X– anions are disordered in four equivalent positions.
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Figure 6. (a) Energy diagram of LED devices with CsxFA1–xPb(Br1–yIy)3 NCs as emissive layers. (b) Current density and radiance versus voltage characteristics of the Device 1. (c) External quantum efficiency versus current density characteristics shown for Devices 1 and 2. (d) Narrowest EL spectra of Device 1 and Device 2. to assess whether the quinary compositions undergo phase peaks appear in a simulation, without having a counterpart separation into respective ternary compounds, we report in the experimental data, thus supporting the the analysis presented in Figure 5c. To investigate the substitutional the pattern of a mixture of FAPbBr3 (13% w/w) and FAPbI3 (87% w/w) cubic phases, and compare it to a solid solution disorder between the FA1–x/Csx and Br1–y/Iy couples, the corresponding site occupancy factors (s.o.f.) were refined model (Figure S16). The result indicates the absence of by the conventional Rietveld method, and the resulting phase segregation. Overall, we highlight that FA+ cations, fully or nearly fully occupying the A‐site of the 3D values (x = 0; y = 0.87) were kept fixed during DSE‐based perovskite framework, systematically favor the formation modeling. The Br/I substitution value is consistent with of a cubic structure [FAPbI3, FAPbBr3, and the herein the PL peak position and with that estimated by X‐ray studied CsxFA1–xPb(Br1–yIy)3], whereas in CsPbX3 NCs, the fluorescence (XRF). The Cs quantity was too low to be detected by X‐ray techniques; however, a small quantity of smaller Cs ions favor the tilting of octahedra and the this cation (
